A prisoner walks around a communal cellblock in the US Navy base at Guantanamo Bay, Cuba. Photo: AFP

For the Past 13 years, al-Qaeda’s most powerful leader in Southeast Asia, Indonesian Riduan Isamuddin, better known as Hambali, has been held in Guantanamo Bay prison, out of public sight but still very much in the minds of militants and security agencies across the region.

Hambali, 52, has a fearsome reputation. The man, known as the Osama bin Laden of Southeast Asia, is thought to have masterminded the Bali bombings that killed more than 200 people in 2002, the Christmas Eve multi-city church bombings in Indonesia that killed 18 in 2000, and the Jakarta Marriott Hotel car bombing that killed 12 in 2003.

Riduan Isamuddin, better known as Hambali, is widely accused of being the operational commander of Jemaah Islamiyah. Photo: AFP

While he has renounced his former ways – telling a review board last August he bore “no ill feelings towards the United States” and “just wanted to move on with life and be peaceful” – that review board this week turned down his application for release, ruling that he continued to be a “significant threat to the security of the United States”.

The board, created by the administration of US President Barack Obama to help winnow down the prison population at Guantanamo as part of a broader effort to close the detention centre, cited Hambali’s “significant role in major terrorist attacks,” as well as a failure to show remorse as factors in its decision.

The decision will be a relief for Asian security agencies, who warn that if he were to be released his enhanced terrorist credentials would only strengthen his ability to revive a dormant Jemaah Islamiah – the group behind the Bali bombings.

But it comes as a blow to his family, who are losing hope he will ever be released, and is the latest signal that Obama may not be able to deliver his seven-year-old pledge to shut the prison before his term ends next month.

WATCH: Life inside Gauntanamo Bay detention facility

“Of course as his family, we hope he will be released. But whatever happens, it is the will of God,” his younger brother, Kankan Abdul Qodir, 38, told This Week in Asia, adding that his brother had “never been brought to trial in a court to prove all these allegations”.

“What can we do if the US government thinks he is a danger to them? We just have to accept it,” Kankan said.

Even if the camp were to close and the US authorities to relinquish custody of Hambali – born Encep Nurjaman – both Indonesia (where he was born) and Malaysia (where he has residency) have ruled out letting him enter their borders.

“He is just an ordinary person who behaved just like anyone else when he was with us,” said Kankan. “Whatever his activities outside, we do not know. Only God knows the truth.”

US authorities allege that Hambali went to fight the Russian occupation in Afghanistan in 1987, meeting Osama bin Laden and becoming the operational chief for al-Qaeda’s Southeast Asia chapter, Jemaah Islamiah.

But that narrative does not sit well with Kankan, who grew up with Hambali in Cianjur, West Java, Indonesia, an area famous for hosting the Islamist group Darul Islam (House of Islam) and its rebellion that began in the 1940s and was not fully quashed until the 1960s.

Hambali was the second child in a family of 12; Kankan is the ninth. “Hambali is much older than me and I did not see much of him as he left for Malaysia sometime in 1983. The last time I saw him was in 1999,” said Kankan.

According to Kankan, Hambali went to Malaysia in search of work, and settled in Sungai Manggis, Selangor, selling kebabs and medicine; Kankan became a teacher at a madrassa [religious boarding school] and also sells insurance for a living.

Kankan, married with three children, says his mother was shocked when Hambali’s name was linked to terrorism after he was captured in Ayutthaya, Thailand, in 2003 in a joint US-Thai operation.

Hambali was sent to Guantanamo, where he remains today as one of 60 high-value detainees whose future appears in limbo as the US administration grapples over what to do with the facility.

Of the nearly 800 prisoners who have passed through Guantanamo, many have been released without charge, but to close the facility, Obama would have to find a home for the remaining detainees.

Moving them to prisons on US soil might prove politically too tricky, but deporting them would also be problematic, given that many, including Hambali, would not be accepted by their home countries.

According to Human Rights First, 20 of the remaining 60 detainees have been approved for release, a figure that may give the family a sliver of hope beyond the 30-minute Skype calls they are allowed with Hambali once every three months thanks to the International Committee of the Red Cross.

WATCH: Ex-Guantanamo inmates begin new, free lives

“Alhamdulillah [thank God] he looks well and healthy. The last time we spoke with him was in August. The next round should be in November,” said Kankan.“My mother has come to accept the situation now. We believe in God and that has helped us to be patient and to carry on with our lives,” he said.

“Our neighbours and the community around us have given us moral support all this while.”

During his appeal, Hambali expressed hopes to “remarry ” and raise children – Kankan revealed that Hambali’s Malaysian wife, Noralwizah Lee Abdullah, an ethnic Chinese who converted to Islam, had divorced him in 2014.

“It has been a long time since he was gone. They have no children,” said Kankan.

Another of Kankan’s brothers, Rusman Gunawan, also known as Gun Gun, was a member of Hambali’s militant network. Gun Gun was jailed for four years in 2004 for “aiding and abetting” in the 2003 Mariott Hotel attack, but his detention was cut short for good behaviour and he was released in 2006.

US authorities believe that since his release he has joined Islamic State’s Indonesian network, a charge Kankan strenuously denies.

“These are negative accusations against my brother. Gun Gun has never taken part in any organisation following his release. He now owns a shop which sells electronic goods in Bengkulu, Sumatera, and he is doing well.”

Were Hambali, like Gun Gun, to find his way back to the region, Malaysian and Indonesian security officials warn his return could revive a dormant Jemaah Islamiah, weakened by the arrests of numerous leaders in recent years; and galvanise the latest generation of militants in the region.

Ayub Khan Mydin Pitchay, head of counterterrorism for Special Branch, the intelligence arm of the Malaysian Royal Police, said Malaysia would not accept Hambali as he held only permanent residency and not citizenship.

However, if he were somehow to return, the country would face a problem: it does not have the legal tools to detain him for his alleged militant activities since the Internal Security Act has been abolished and replaced by the Security Offences (Special Measures) Act 2012.

“[This act] cannot be used to hold him as Hambali’s wrongdoings were committed from 1985 to 2003. [It] cannot be applied retroactively,” said Ayub.

This fact, together with Hambali’s extensive network of contacts, has left intelligence agencies fearing the potential for disaster if he is ever released.

Ayub warned Hambali would “roam Southeast Asia” and become “active again in terrorist activities” if freed.

“Many of Jemaah Islamiyah’s [leaders] were nurtured by Hambali and [Malaysian] Yazid Sufaat,” said Ayub, referring to the jailed militant who hosted two of the September 11 hijackers at his apartment in 2000.

“Hambali has a great capacity to recruit people… he is charismatic, has many contacts with top al-Qaeda leaders, and is very capable of organising large terrorist attacks against US interests in Singapore, Thailand, Indonesia.”

A senior Indonesian security official said a freed Hambali would revive Jemaah Islamiah. “It would be a disaster for Indonesia if Hambali returns as he is very dangerous,” said the official.

“He would become a hero in Indonesia and I would expect all radical groups to rally round him. His reputation as a global jihadist has only earned him greater acknowledgement among the radicals.”